1. Field of the Invention
The present invention relates to communication networks in general and in particular to communication networks comprising of one or more switches interconnecting Local Area Networks (LANs).
2. Prior Art
The use of switches for interconnecting LANs are well known in the prior art. A conventional switch includes at least two ports coupled through one or more frame processing modules to a switch fabric. The switch fabric provides the interconnection between the ports. Each port is connected to a LAN segment that has a shared transmission medium to which a plurality of stations are connected. Even though the stations co-habitate on shared media, when stations on different ports are communicating, the transmission is point-to-point. As a consequence, the switch provides more bandwidth for stations communicating from different LAN segments.
Even though the switch provides more bandwidth to stations, it presents unique problems that have to be addressed in order for the interconnected LANs to operate efficiently. One of the problems is that each port has to function as if it were a station on the LAN that is connected to it and at the same time function as a part of the switch. When it functions as a station on the LAN, the port is obliged to practice the protocol associated with the LAN. For example, if a Token Ring LAN Segment is connected to the port, the port must operate according to the Token Ring Protocols set forth by the IEEE 802.5 Standard. Likewise, the port must behave in accordance with MAC bridge standard IEEE 802.1D.
Even though none of the above standards mandates setting of A/C bits by a switch port (details set forth below) in an LLC frame, certain customs and usage in the Token Ring environment requires setting of these bits by stations to which the frame is addressed and is copied. In particular, the device drivers in the protocol stack of host stations rely on the setting of the A/C bits to determine if a session is being maintained. If the A/C bits are set to logical "1", the frame has been received by the end station. If the A/C bits are set to logical "0", the frame was not delivered. With this reliance, there is a need to set the A/C bits or else the protocol stack does not function effectively.
On the other hand, if the switch port sets the A/C bits in all frames, destination stations on receiving frames with the A/C bits set to logical "1" would issue error reports. These error reports require action which would necessarily slow down the ring.
One obvious solution is for the switch port to set A/C bits only in frames that it forwards. The problem with this solution is that the switch maintains a database of thousands of addresses. It is not possible to examine all the addresses to determine if the frame is being forwarded to set the A/C bits in a timely manner. Even if it were possible, the device, such as a high speed CAM (Contents Address Memory) for performing the examination would be extremely expensive and unnecessarily increase the product cost.
As a consequence, there is a need for a device and method that is low cost and effective in controlling the A/C bits at switch ports.